Host defense against pathogen infection is critically dependent on the differentiation of effector programs associated with specialized populations of innate and adaptive lymphocytes. Each program is orchestrated by a signature transcription factor, but the mechanisms of gene regulation vary and, in some cases, depend on complex interactions mediated by associated nuclear proteins. The BTB-ZF transcription factors PLZF and Bcl- 6 recruit a well-defined co-repressor complex through their BTB domain in order to induce chromatin modifications at sequence-specific target sites bound by their zinc fingers. PLZF directs the developmental acquisition of innate-like effector properties by microbial lipid-specific NKT cells, whereas Bcl-6 directs the adaptive differentiation of the T follicular helper (TFH) and the germinal center (GC) B cells. Both factors are also well-known oncogenes involved in leukemias and lymphomas. Based on extensive preliminary studies, we propose to test the hypothesis that PLZF and Bcl6 recruit a novel partner, the E3 ubiquitin ligase cullin 3 (Cul3), which exerts a critical role on their transcriptional program through ubiquitination of associated nuclear protein complexes. The objective of this application is to characterize the association of Cul3 with PLZF and its impact on the corresponding lymphocyte effector program. The rationale for this project is that it will provide unprecedented insights into the molecular details of regulation of this host defense program. In turn, these insights will allow manipulation of these processes for therapeutic benefit. The proposed hypothesis will be tested by pursuing the following specific aims: 1) to characterize the dysregulation of effector programs in mice lacking Cul3 in their lymphoid compartments; 2) to characterize the binding and transport of Cul3; 3) to characterize the targets of Cul3. The data will be integrated to develop a basic understanding of the role of Cul3 in the transcriptional program directed by PLZF. The proposal is innovative because it explores the function of a novel partner of a family of transcription factors that regulate vital properties of host defense against pathogens. The proposed research is significant because it will enhance our understanding of essential lymphocyte effector programs and generate new concepts and tools that will make it possible to develop agents that target specific regulatory components of immune responses.